Conventional analog telephone line-interface circuits, also known as analog port circuits, require a 48VDC power supply for operation and for reliable signaling on long subscriber loops (telephone lines). Long loops have a high resistance relative to short loops, and therefore require a relatively high voltage to drive them. The circuit which couples the DC power to the telephone line is known as a battery-feed circuit. Even though battery-feed circuits commonly employ current-limiting and limit loop current to 42mA, 2W of power can be dissipated by the line-interface circuit. This high power dissipation limits the number of line-interface circuits that can be integrated on a single integrated-circuit device (a "chip"), as well as the number of telephone lines that can be served by a single 48V power supply.
To reduce power dissipation, the art has employed dual-supply line-interface circuits. These circuits employ a second power supply having a voltage lower than the high-voltage (48V) power supply, for powering short subscriber loops.